Carrier sense multiple access with collision avoidance (CSMA/CA) is a multiple access technique used, for example, in the WLAN (wireless local area network) standard IEEE 802.11ac.
According to the principles of CSMA/CA, a device that has anything to transmit must first sense the communication channel and determine it to be free during at least a sensing period before initiating transmission. If the device determines the communication channel to be busy, it continues the sensing process until the communication channel is free during at least the sensing period. Typically, the device may apply a back-off period (see below) before continuing the sensing process when it determines the communication channel to be busy. Once a device gets hold of the communication channel, it may perform its transmission (e.g. transmit data as OFDM symbols).
If two (or more) devices happen to initiate transmission at the same time (both having determined the communication channel to be free during their respective sensing periods as explained above) a collision occurs. When a probable collision is detected (typically by detecting a packet loss), each device involved in the collision defers its transmission associated activity during a back-off period. When the back-off period has elapsed the device iterates the procedure, starting with the sensing of the communication channel, until the transmission is complete.
To reduce the probability of a second collision, the back-off period of a device is a randomly chosen time period whose length is chosen according to a distribution within a certain range, called the contention window (CW).
Different users (e.g. different devices and/or different services used by a device) may have different latency requirements. The latency requirements may typically relate to the maximum acceptable time from a need to transmit until a successful transmission is accomplished. Such a maximum acceptable time will, inherently include a time for accessing the communication channel.
To exemplify how different latency requirements may be handled, the various types of access categories (AC) introduced in WLAN standards may be considered. These include high priority services, such as voice (AC_VO), using a shorter contention window (i.e. a shorter back-off period on the average) and a shorter sensing period (defined by the arbitration inter frame space (AIFS)) than low-priority services. This enables that the high priority services, when they contend with low-priority services for the communication channel, have a higher probability of getting hold of the communication channel than low-priority services and that they, consequently, experience shorter delays.
Even so, the experienced delay from the time when a need for transmission arises to the time of accomplished transmission may not be acceptably short for some applications. For example, other time-critical applications are emerging that would require even shorter delays than the current high priority services such as voice.
Therefore, there is a need for alternative approaches for CSMA/CA systems to accommodate users with different latency requirements.